1. Field of the Invention
The invention relates to a vacuum booster for reducing the force required for operating a pedal-operated brake system of automobiles and, more particularly, to a control valve mechanism which is operated by a push rod linked to the brake pedal which controls a pressure differential between front and rear chambers in the housing.
2. Description of the Prior Art
A vacuum booster for automobiles of a prior type is described, for example, in Japanese Utility Model Laid-Open No. 57-92569.
Referring to FIG. 4, in the aforementioned prior arrangement there is provided a vacuum booster which includes a control valve mechanism comprising a valve plunger 53 connected with a front end portion of a push rod 51. The valve plunger 53 is slidably disposed within a power piston 52. In the rest state, a control valve element 54 contacts a seat surface 53a of the rear end portion of the valve plunger 53. As a result of the displacement of the valve plunger 53, the control valve element 54 contacts a valve seat 52a of the power piston 52. Thereafter, the valve plunger 53 separates from the control valve element 54, whereupon a pressure differential is produced between front and rear chambers of the power piston 52 so that the desired operational pressure force is produced by the booster push rod 51.
In this prior arrangement, the seat surface 53a of the rear end portion of the valve plunger 53 is spherical so as to prevent performance degradation of the vacuum booster due to degradation of contact between the valve seat 52a of the power piston 52 and the oontrol valve element 54 on account of an inclination of the valve plunger 53 and the control valve element 54 arising from clearance between the valve plunger 53 and the power piston 52.
In this prior arrangement, however, because the seat surface 53a of the rear end portion of the valve plunger is spherical, it is necessary to form a circular projection on the thickness portion of a membrane required for shaping the control valve member 54 in order to raise the surface pressure of the seal surface. As a result, the thickness of the circular projection will be large, and the seat surface of the valve plunger in contact with the control valve element in the normal condition elastically deforms the circular projection to a large extent so as to assure that the communication between the atmosphere and the chamber in the housing is interrupted. Therefore, there will be an increase in the deformation of the circular projection in a needful stroke of the brake pedal which is required to operate the vacuum booster by separating from the surface of the valve plunger. Consequently, the prior arrangement has a drawback which has a bad influence on the feeling of the brake operation.